The present invention relates to spindle-shaped magnetic alloy particles for magnetic recording, and a magnetic recording medium, and more particularly, to a high-density magnetic recording medium having an excellent output characteristics in a short wavelength region, a considerably reduced noise and an excellent storage stability; spindle-shaped magnetic alloy particles containing Fe and Co as main components, which are contained in the magnetic recording medium and are fine particles exhibiting not only a high coercive force, an adequate saturation magnetization value and an excellent oxidation stability in spite of a small crystallite size, but also a small rotational hysteresis integral value when formed into a coating film; and a high-density magnetic recording medium having an excellent output characteristics in a short wavelength region, a considerably reduced noise and an excellent storage stability.
With recent development of miniaturized and lightweight magnetic recording and reproducing apparatuses for use in audio, video and computer as well as increase in recording time and memory capacity thereof, magnetic recording media used therefor have increasingly required to have a high performance and a high recording density.
In particular, in the field of computer tapes, with rapid development of high performance computers, it has been strongly required to enhance a memory capacity thereof for achieving miniaturization and large capacity thereof.
Namely, the magnetic recording media have been required to exhibit a high recording density, high output characteristics and improved frequency characteristics, especially excellent output characteristics in a short wavelength region. For this purpose, it has been required to reduce a noise of the magnetic recording media, and enhance a coercive force Hc thereof.
Also, in recent magneto resistive-type head as a reproduction head for computer tapes instead of conventional induction-type magnetic heads. Since the MR head can readily produce a considerably high reproduction output as compared to the conventional induction-type magnetic heads, it has been expected to further improve a high-density recording performance of the magnetic recording media.
In particular, the MR head is free from impedance noise due to use of induction coil and, therefore, is capable of reducing a system noise such as device noise to a large extent, resulting in reduced noise and high C/N ratio of the magnetic recording media. Accordingly, it has been strongly required to further reduce the noise of the magnetic recording media as compared to conventional ones.
In addition, in order to achieve a high-density recording, especially reduce a recording wavelength, it has been required to narrow a magnetization transition region and sharpen a digital signal reproduction waveform from the standpoints of high output and low noise. For this purpose, it has also been desired to approach the magnetization reversal to coherent rotation relative to a magnetic field and lessen the width of magnetization reversal.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic alloy particles containing iron as a main component have been noticed, because such particles can show a high coercive force and a large saturation magnetization ("sgr"s) as compared to those of conventional magnetic iron oxide particles. Further, magnetic alloy particles containing iron as a main component have been already used as magnetic particles for external memory devices such as computer tapes, e.g., DDS, DLT and TRAVAN.
Therefore, it has been strongly required to further improve properties of the magnetic alloy particles containing iron as a main component in order to satisfy the above requirements for magnetic recording media.
Specifically, in order to obtain magnetic recording media having a high coercive force, a less noise and a small width of magnetization reversal, it has been strongly required to provide fine magnetic alloy particles containing iron as a main component which are fine particles and are capable of not only exhibiting a small crystallite size, a high coercive force and an excellent dispersibility, but also having a magnetization reversal mechanism for ensuring substantially coherent rotation of the magnetization relative to a magnetic field.
As to the reduction in particle size of the magnetic alloy particles, in Japanese Patent Application Laid-Open (KOKAI) No. 2000-251243, it is described that xe2x80x9c. . . When the size of a magnetic particles used becomes as large as compatible with a length of a recording region for signals, a clear magnetization transition region is no longer available, so that it becomes substantially impossible to record signals thereon. For this reason, it has been longtime demanded to provide the fine particles for achieving high-density recording upon use . . . xe2x80x9d Thus, in order to obtain magnetic recording media having a high output characteristics in a short wavelength region, and a less noise, it is required to reduce the particle size of the magnetic alloy particles for obtaining fine particles, i.e., reduce the major axis diameter thereof.
Also, as to the crystallite size of the magnetic alloy particles, in Japanese Patent Application Laid-Open (KOKAI) No. 7-126704 (1995), it is described that xe2x80x9c. . . In order to reduce the noise level due to magnetic recording media, it is also effective to reduce the X-ray-measured size to as low a level as possible . . . xe2x80x9d. Thus, in order to obtain magnetic recording media having a less noise, the magnetic alloy particles containing iron as a main component have been strongly required to have a smaller crystallite size D110.
In addition, in order to further reduce the noise of the magnetic recording media, it is insufficient to lessen merely the major axis diameter and crystallite size of the magnetic alloy particles. For this purpose, it has been strongly required to clearly determine what factors should be concerned with the noise to be reduced.
In particular, there has been recently studied the activation volume obtained by measuring a time decay of magnetization due to thermal fluctuation and magnetic after-effect. On the basis of such studies, the reduction in noise of magnetic recording media has been attempted by optimizing the activation volume.
Also, it has been required to approach a magnetization reversal mode of the magnetic alloy particles to coherent rotation.
However, it is very difficult to obtain magnetic alloy particles containing Fe and Co as main components, which are fine particles, and exhibit a small crystallite size, a high coercive force and a magnetization reversal mode close to coherent rotation, because of its production process.
The above fact is explained below.
That is, the magnetic alloy particles containing iron as a main component have been generally produced by heat-reducing in a reducing gas atmosphere (i) spindle-shaped goethite particles obtained by passing an oxygen-containing gas such as air through an aqueous solution containing an iron-containing precipitate produced by reacting an aqueous ferrous salt solution with an aqueous alkali solution to conduct an oxidation reaction thereof, (ii) spindle-shaped hematite particles obtained by heat-dehydrating the spindle-shaped goethite particles, or (iii) spindle-shaped particles obtained by incorporating different elements other than iron into these particles.
As to the relationship between crystallite size and coercive force, in Japanese Patent Application Laid-Open (KOKAI) No. 4-61302 (1992), it is described that xe2x80x9c. . . There is a tendency that the smaller the crystallite size, the lower the coercive force. Therefore, . . . it has been strongly required to provide magnetic particles exhibiting a small crystallite size while maintaining as high a coercive force as possible . . . xe2x80x9d. Thus, in the case of the spindle-shaped magnetic alloy particles, the crystallite size and the coercive force thereof have a reverse interrelation to each other. Therefore, it is extremely difficult to obtain magnetic alloy particles satisfying both a small crystallite size and a high coercive force.
In addition, in the consideration of good oxidation stability of the magnetic alloy particles, it is necessary to fully enhance the reduction percentage of the particles by increasing a heat-reducing temperature to as high a value as possible. However, when the heat-reducing temperature is elevated, the starting particles tend to suffer from shape destruction, resulting in deteriorated coercive force of the obtained magnetic alloy particles. Further, since the heat-reduction conditions such as atmosphere and temperature are very severe, the obtained spindle-shaped magnetic alloy particles tend to suffer from sintering within or between the particles.
In recent years, the particle size of magnetic alloy particles is more and more reduced in order to impart a high coercive force thereto. For this reason, the particle size of the starting particles is also reduced. However, in the case where the starting particles are fine particles having a particle size of not more than 0.15 xcexcm, there is such a remarkable tendency that the particles suffer from shape destruction upon the heat-reduction process. Such shape-destroyed magnetic alloy particles cannot exhibit a high coercive force because of shape anisotropy thereof, and are deteriorated in size distribution. Further, the reduction in particle size of the magnetic alloy particles causes a rapid deterioration in oxidation stability thereof. In addition, when such fine particles are used for the production of magnetic recording media, the dispersibility of the fine particles in vehicles tends to be deteriorated because of the increase in intermolecular force between the particles or the increase in magnetic cohesive force therebetween when kneaded with a binder and dispersed in vehicles. As a result, a magnetic coating film produced from such fine particles is deteriorated in squareness, so that it becomes difficult to obtain magnetic recording media exhibiting an excellent SFD.
Also, although the magnetization reversal mode of the magnetic alloy particles can approach to coherent rotation by enhancing a single crystal growth thereof, there arises a problem that the enhanced single crystal growth also causes increase in crystallite size of the magnetic alloy particles. Therefore, it has been difficult to achieve both the requirements of approaching the magnetization reversal mode of the magnetic alloy particles to coherent rotation, and lessening the crystallite size thereof.
The method of obtaining magnetic recording media exhibiting a high output characteristics and a low noise by improving properties of the magnetic alloy particles has been described in Japanese Patent Application Laid-Open (KOKAI) Nos. 8-171718 (1996), 9-22522 (1997), 9-22523 (1997), 9-106535 (1997) and 10-302243 (1998), etc.
At present, it has been strongly demanded to provide spindle-shaped magnetic alloy particles not only having a high coercive force and an adequate saturation magnetization value in spite of a crystallite size as small as especially not more than 160 xc3x85, but also exhibiting a magnetization reversal mode close to coherent rotation. However, such magnetic alloy particles fully satisfying the aimed properties have not been obtained.
That is, in Japanese Patent Application Laid-Open (KOKAI) No. 8-171718 (1996), although the ratio of coercive force Hc to an anisotropy field HK of a magnetic recording medium is specified therein, it is still insufficient to reduce a noise. In addition, properties of magnetic alloy particles required for reducing the noise, have not been fully studied.
In Japanese Patent Application Laid-Open (KOKAI) Nos. 9-22522 (1997) and 9-22523 (1997), the number of crystallites of magnetic alloy particles is specified, and in Japanese Patent Application Laid-Open (KOKAI) No. 9-106535 (1997), there are specifically described the number of crystallites and crystallinity of magnetic alloy particles as well as the magnetization reversal mode. However, these specified properties of the magnetic alloy particles cannot satisfy the requirement for reducing the noise, and are still insufficient from the standpoint of bringing the magnetization reversal mode into coherent rotation.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 10-302243 (1998), there is specifically described a magnetization reversal volume of magnetic recording media which is obtained from a frequency-dependence of the coercive force. The specified property is still insufficient to fully reduce the noise. In addition, there are no descriptions concerning properties of magnetic alloy particles required for reducing the noise.
In xe2x80x9cTechnical Report of the Institute of Electronics Information and Communication Engineersxe2x80x9d, MR97-22, pp. 29 to 34 (1997-07), there is described the relationship between activation volume and actual volume of magnetic recording media using magnetic alloy particles. However, since the magnetic recording media have a large activation volume, it is not suitable to fully reduce the noise. In addition, any properties of magnetic alloy particles required for reducing the noise are not fully studied.
Also, in xe2x80x9cJournal of Magnetism and Magnetic Materialsxe2x80x9d, 193, pp. 314 to 317 (1999), there is described the activation volume of magnetic alloy particles. However, since the magnetic alloy particles have a large crystallite size, it is not possible to fully reduce the noise.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by heating spindle-shaped goethite particles containing cobalt in an amount of 20 to 50 atm % (calculated as Co) based on whole Fe and having an average major axis diameter of 0.05 to 0.15 xcexcm, or spindle-shaped hematite particles obtained by heat-dehydrating the above spindle-shaped goethite particles as starting particles, to a temperature of 300 to 600xc2x0 C. in an inert gas atmosphere, and after replacing the inert gas atmosphere with a reducing gas atmosphere, heat-reducing the above heat-treated particles at a temperature of 300 to 600xc2x0 C., thereby obtaining spindle-shaped magnetic alloy particles;
subjecting the obtained spindle-shaped magnetic alloy particles to surface oxidation at a temperature of 80 to 200xc2x0 C. in an oxygen-containing inert gas atmosphere, thereby controlling the saturation magnetization value of the spindle-shaped magnetic alloy particles to 85 to 135 Am2/kg;
heating the obtained spindle-shaped magnetic alloy particles to a temperature higher by not less than 50xc2x0 C. than the above heat-reducing temperature which are between 400 to 700xc2x0 C., in an inert gas atmosphere, and after replacing the inert gas atmosphere with a reducing gas atmosphere, heat-reducing the obtained particles again at a temperature of 400 to 700xc2x0 C.; and
subjecting the obtained spindle-shaped magnetic alloy particles again to surface oxidation at a temperature of 40 to 160xc2x0 C. in an inert gas atmosphere containing water vapor in an amount of 5 to 10 g/cm3 and oxygen,
a magnetic recording medium produced using the thus obtained spindle-shaped magnetic alloy particles exhibits not only an excellent output characteristics in a short wavelength region and a considerably reduced noise, but also an excellent storage stability. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide spindle-shaped magnetic alloy particles which are fine particles, and exhibit not only a high coercive force and an adequate saturation magnetization vale in spite of a small crystallite size, but also a small rotational hysteresis integral value.
Another object of the present invention is to provide a high-density magnetic recording medium capable of exhibiting not only an excellent output characteristics in a short wavelength region and a considerably reduced noise, but also an excellent storage stability.
To accomplish the aims, in a first aspect of the present invention, there are provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3.
In a second aspect of the present invention, there are provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.08 xcexcm; an average minor axis diameter of 0.008 to 0.020 xcexcm; an aspect ratio (average major axis diameter/average minor axis diameter) of 3:1 to 8:1; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 110 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3.
In a third aspect of the present invention, there are provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3; and a rotational hysteresis integral value (Rh) of not more than 1.0.
In a fourth aspect of the present invention, there are provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a saturation magnetization value of 100 to 150 Am2/kg; a crystallite size of 100 to 160 xc3x85; an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3; and a rotational hysteresis integral value (Rh) of not more than 1.0.
In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate, and a magnetic layer formed on the non-magnetic substrate, which comprises a binder resin and the spindle-shaped magnetic alloy particles containing Fe and Co as main components, which have a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3.
In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate, and a magnetic layer formed on the non-magnetic substrate, which comprises a binder resin and the spindle-shaped magnetic alloy particles containing Fe and Co as main components, which have a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3,
the magnetic recording medium having a coercive force Hc of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of not less than 0.82; an orientation degree of not less than 2.0; an oxidation stability xcex94Bm of less than 8%; and a surface roughness Ra of not more than 8 nm.
In a seventh aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate, and a magnetic layer formed on the non-magnetic substrate, which comprises a binder resin and the spindle-shaped magnetic alloy particles containing Fe and Co as main components, which have a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3,
said magnetic recording medium having a coercive force Hc of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of not less than 0.82; an orientation degree of not less than 2.0; an oxidation stability xcex94Bm of less than 8%; and a surface roughness Ra of not more than 8 nm.
In an eighth aspect of the present invention, there is provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 45 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.08 xcexcm; an average minor axis diameter of 0.008 to 0.020 xcexcm; an aspect ratio (average major axis diameter/average minor axis diameter) of 3:1 to 8:1; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 110 to 160 xc3x85; and an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3,
the magnetic recording medium having a coercive force Hc of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of not less than 0.82; an orientation degree of not less than 2.0; an oxidation stability xcex94Bm of less than 8%; and a surface roughness Ra of not more than 8 nm.
In a ninth aspect of the present invention, there is provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; an average minor axis diameter of 0.008 to 0.020 xcexcm; an aspect ratio (average major axis diameter/average minor axis diameter) of 3:1 to 8:1; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a crystallite size of 100 to 160 xc3x85; an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3; and a rotational hysteresis integral value (Rh) of not more than 1.0,
the magnetic recording medium having a coercive force Hc of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of not less than 0.82; an orientation degree of not less than 2.0; an oxidation stability xcex94Bm of less than 8%; and a surface roughness Ra of not more than 8 nm.
In a tenth aspect of the present invention, there is provided spindle-shaped magnetic alloy particles containing Fe and Co as main components, having a cobalt content of 20 to 50 atm %, calculated as Co, based on whole Fe; an average major axis diameter (L) of 0.03 to 0.10 xcexcm; an average minor axis diameter of 0.008 to 0.020 xcexcm; an aspect ratio (average major axis diameter/average minor axis diameter) of 3:1 to 8:1; a coercive force value of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a saturation magnetization value of 100 to 150 Am2/kg; a crystallite size of 100 to 160 xc3x85; an activation volume (Vact) of 0.01 to 0.07E-4 xcexcm3; and a rotational hysteresis integral value (Rh) of not more than 1.0,
the magnetic recording medium having a coercive force Hc of 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of not less than 0.82; an orientation degree of not less than 2.0; an oxidation stability xcex94Bm of less than 8%; and a surface roughness Ra of not more than 8 nm.
The present invention will be described in detail below.
First, the spindle-shaped magnetic alloy particles of the present invention are described.
The spindle-shaped magnetic alloy particles of the present invention contain cobalt in an amount of usually 20 to 50 atm %, preferably 20 to 45 atm % (calculated as Co) based on whole Fe. When the cobalt content is less than 20 atm %, the obtained particles cannot be sufficiently improved in oxidation stability, and can not show a high coercive force. When the cobalt content is more than 50 atm %, the obtained particles are deteriorated in saturation magnetization value and coercive force. In addition, the used of a high cobalt content is also undesirable from economical viewpoint.
The spindle-shaped magnetic alloy particles of the present invention have an average major axis diameter of usually 0.03 to 0.10 xcexcm, preferably 0.03 to 0.095 xcexcm, more preferably 0.03 to 0.08 xcexcm. When the average major axis diameter is less than 0.03 xcexcm, the obtained spindle-shaped magnetic alloy particles suffer from considerable deterioration in saturation magnetization value and oxidation stability, and simultaneously can not show a high coercive force. Further, since the time decay in recording quality due to thermal fluctuation is not ignorable, the obtained particles having such a small average major axis diameter cannot be used as recording media. When the average major axis diameter is more than 0.10 xcexcm, such spindle-shaped magnetic alloy particles can not fully exhibit the aimed high output characteristics in a short wavelength region and a low noise, and are also deteriorated in coercive force.
The spindle-shaped magnetic alloy particles of the present invention have an average minor axis diameter of usually 0.008 to 0.020 xcexcm. When the average minor axis diameter is more than 0.020 xcexcm, the obtained spindle-shaped magnetic alloy particles may be deterioration in coercive force and anisotropy field. Further, it may become difficult to obtain spindle-shaped magnetic alloy particles satisfying all requirements for crystallite size, activation volume and rotational hysteresis integral vale. When the average minor axis diameter is less than 0.008 xcexcm, the obtained spindle-shaped magnetic alloy particles may suffer from considerable deterioration in saturation magnetization value and oxidation stability, and it may be difficult show a high coercive force. Further, since the time decay in recording quality to thermal fluctuation is not ignorable, the obtained particles having such a small average minor axis diameter may not be used as recording media.
The spindle-shaped magnetic alloy particles of the present invention have an aspect ratio (average major axis diameter/average minor axis diameter) of preferably 3:1 to 8:1.
The spindle-shaped magnetic alloy particles of the present invention have a crystallite size D110 of usually 100 to 160 xc3x85, preferably 100 to 155 xc3x85. When the crystallite size D110 is more than 160 xc3x85, the aimed reduction in noise in a short wavelength region may not be sufficiently achieved. When the crystallite size D110 is less than 100 xc3x85, the obtained spindle-shaped magnetic alloy particles may suffer from considerable deterioration in saturation magnetization value and oxidation stability, and simultaneously can not show a high coercive force. Further, since the time decay in recording quality due to thermal fluctuation is not ignorable, the obtained particles having such a small crystallite size may not be used as recording media.
The spindle-shaped magnetic alloy particles of the present invention have an activation volume (Vact) of usually 0.01 to 0.07E-4 xcexcm3, preferably 0.015 to 0.07E-4 xcexcm3. When the activation volume is more than 0.07E-4 xcexcm3, the aimed reduction in noise in a short wavelength region may not be sufficiently achieved. When the activation volume is less than 0.01E-4 xcexcm3, the obtained spindle-shaped magnetic alloy particles may suffer from considerable deterioration in saturation magnetization value and oxidation stability, and simultaneously can not show a high coercive force. Further, since the time decay in recording quality due to thermal fluctuation is not ignorable, the obtained particles having such a small activation volume may not be used as recording media.
The spindle-shaped magnetic alloy particles of the present invention have a rotational hysteresis integral (Rh) value of usually not more than 1.0, preferably not more than 0.95. When the rotational hysteresis integral (Rh) value is more than 1.0, the high output characteristics in a short wavelength region and the reduction in noise may not be achieved.
In addition, the spindle-shaped magnetic alloy particles of the present invention have a BET specific surface area value of preferably 40 to 75 m2/g, more preferably 45 to 75 m2/g. When the BET specific surface area is more than 75 m2/g, it may be difficult to disperse the obtained spindle-shaped magnetic alloy particles in vehicles upon producing a coating material therefrom. When the BET specific surface area is less than 40 m2/g, it is difficult to obtain spindle-shaped magnetic alloy particles capable of satisfying all requirements for crystallite size, activation volume and major axis diameter.
The spindle-shaped magnetic alloy particles of the present invention have a coercive force Hc of usually 159.2 to 238.7 kA/m (2,000 to 3,000 Oe), preferably 167.1 to 222.8 kA/m (2,100 to 2,800 Oe). When the coercive force is more than 238.7 kA/m, the recording head is saturated, thereby failing to obtain the aimed high output characteristics in a short wavelength region. When the coercive force is less than 159.2 kA/m, it is difficult to obtain a fully high output characteristics in a short wavelength region.
The spindle-shaped magnetic alloy particles of the present invention preferably have a saturation magnetization value ("sgr"s) of usually 100 to 150 Am2/kg (100 to 150 emu/g), preferably 100 to 140 Am2/kg (100 to 140 emu/g). When the saturation magnetization value is more than 150 Am2/kg, the increase in noise may be caused. In particular, when an MR head is used as a reproduction head, excessive residual magnetization may be produced, resulting in unnecessary increase in noise and, therefore, deterioration in C/N ratio. When the saturation magnetization value is less than 100 Am2/kg, the obtained magnetic alloy particles may undergo problems such as deteriorated coercive force and wide coercive force distribution.
The spindle-shaped magnetic alloy particles of the present invention have a squareness ("sgr"r/"sgr"s) of 0.52 to 0.55.
The spindle-shaped magnetic alloy particles of the present invention have an anisotropy field (Hk) of usually 477.5 to 636.6 kA/m (6,000 to 8,000 Oe), preferably 517.3 to 636.6 (6,500 to 8,000 Oe). It is difficult to industrially produce such magnetic alloy particles having an anisotropy field (Hk) of more than 636.6 kA/m (8,000 Oe). When the anisotropy field (Hk) is less than 477.5 kA/m (6,000 Oe), it is difficult to attain the aimed high output characteristics in a short wavelength region.
The spindle-shaped magnetic alloy particles of the present invention have an oxidation stability (xcex94"sgr"s) of saturation magnetization of preferably not more than 15%, more preferably not more than 13%, still more preferably not more than 10%.
Next, the process for producing the magnetic recording medium according to the present invention is described.
The magnetic recording medium of the present invention comprises a non-magnetic substrate, and a magnetic recording layer formed on the non-magnetic substrate, which comprises the spindle-shaped magnetic alloy particles of the present invention and a binder resin.
As the non-magnetic substrate, there may be used those presently used in ordinary magnetic recording media, e.g., films of synthetic resins such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonates, polyethylene naphthalate, polyamides, polyamideimides and polyimides, foils and plates of metals such as aluminum and stainless steel, and various papers. The thickness of the non-magnetic substrate may be varied depending upon materials thereof, and is preferably 1.0 to 300 xcexcm, more preferably 2.0 to 50 xcexcm.
More specifically, in the case of magnetic discs, a non-magnetic substrate thereof may be usually made of polyethylene terephthalate, and has a thickness of usually 50 to 300 xcexcm. In the case of magnetic tapes, a non-magnetic substrate thereof may be made of polyethylene terephthalate, polyethylene naphthalate, polyamide or the like; and the polyethylene terephthalate substrate has a thickness of usually 3 to 100 xcexcm; the polyethylene naphthalate substrate has a thickness of usually 3 to 50 xcexcm, and the polyamide substrate has a thickness of usually 2 to 10 xcexcm.
As the binder, there may be used those presently used for the production of ordinary magnetic recording media, e.g., vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivatives such as nitrocellulose, polyester resin, synthetic rubber-based resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron beam-curable acrylic urethane resin, or mixtures thereof.
Also, the binder resin may contain polar groups such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2, wherein M represents hydrogen, Na or K.
The magnetic recording layer formed on the non-magnetic substrate has a thickness of usually 0.01 to 5.0 xcexcm. When the thickness is less than 0.01 xcexcm, it may tend to be difficult to form a uniform magnetic recording layer because of coating unevenness or the like. When the thickness is more than 5.0 xcexcm, it may tend to be difficult to obtain the aimed electromagnetic performance because of adverse influence of demagnetizing field.
The amount of the spindle-shaped magnetic alloy particles contained in the magnetic recording layer is usually 5 to 2,000 parts by weight based on 100 parts by weight of the binder resin.
Meanwhile, the magnetic recording layer may further contain, if required, known additives ordinarily used in magnetic recording media such as lubricants, abrasives, anti-static agents or the like in an amount of usually 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
The magnetic recording medium obtained using the spindle-shaped magnetic alloy particles according to the present invention exhibits a coercive force Hc of usually 159.2 to 238.7 kA/m (2,000 to 3,000 Oe); a squareness (Br/Bm) of usually not less than 0.82, preferably not less than 0.85; an SFD of usually not more than 0.60, preferably not more than 0.50, more preferably not more than 0.45; an orientation degree of usually not less than 2.0, preferably not less than 2.3; an oxidation stability xcex94Bm of usually less than 8%, preferably less than 5%; and a surface roughness Ra of usually not more than 8 nm, preferably not more than 5 nm.
In the magnetic recording medium of the present invention, a non-magnetic undercoat layer containing non-magnetic particles and a binder resin may be disposed between the non-magnetic substrate and the magnetic recording layer.
As the non-magnetic particles for the non-magnetic undercoat layer, there may be used non-magnetic inorganic particles ordinarily used in a non-magnetic undercoat layer of magnetic recording media. Specific examples of the non-magnetic particles may include particles of hematite, iron oxide hydroxide, titanium oxide, zinc oxide, tin oxide, tungsten oxide, silicon dioxide, xcex1-alumina, xcex2-alumina, xcex3-alumina, chromium oxide, cerium oxide, silicon carbide, titanium carbide, silicon nitride, boron nitride, calcium carbonate, barium carbonate, magnesium carbonate, strontium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, barium titanate or the like. These non-magnetic particles may be used alone or in combination of any two or more thereof. Among these non-magnetic particles, particles of hematite, iron oxide hydroxide and titanium oxide, etc., are preferred.
Upon the production of a non-magnetic coating composition, in order to improve a dispersibility thereof in vehicles, the surface of the non-magnetic particles may be coated, if required, with hydroxides of aluminum, oxides of aluminum, hydroxides of silicon, oxides of silicon or the like. In addition, in order to improve properties of the obtained magnetic recording medium such as light transmittance, surface resistivity, mechanical strength, surface smoothness, durability or the like, various elements such as Al, Ti, Zr, Mn, Sn, Sb, etc., may be incorporated into the non-magnetic particles according to requirements.
The non-magnetic particles may have various shapes, and may include, for example, granular particles such as spherical particles, granulated particles, octahedral particles, hexahedral particles and polyhedral particles; acicular particles such as needle-like particles, spindle-shaped particles and rice ball-shaped particles; plate-shaped particles; or the like. In the consideration of good surface smoothness of the obtained magnetic recording medium, the non-magnetic particles preferably have an acicular particles.
The non-magnetic particles has an average particle diameter of usually 0.01 to 0.3 xcexcm, and may be usually of granular, acicular or plate shape.
The acicular non-magnetic particles have an aspect ratio of usually 2:1 to 20:1, and the plate-shaped non-magnetic particles have a plate ratio (average plate surface diameter/average thickness) of usually 2:1 to 50:1.
The non-magnetic undercoat layer preferably has a thickness of 0.2 to 10.0 xcexcm. When the thickness of the non-magnetic undercoat layer is less than 0.2 xcexcm, it is difficult to improve the surface roughness of the non-magnetic substrate.
As the binder resin for the non-magnetic undercoat layer, there may be used those binder resins exemplified above for the production of the magnetic recording layer.
The amount of the non-magnetic particles contained in the non-magnetic undercoat layer is usually 5 to 2,000 parts by weight based on 100 parts by weight of the binder resin.
Meanwhile, the non-magnetic undercoat layer may further contain, if required, known additives ordinarily used in magnetic recording media such as lubricants, abrasives, anti-static agents or the like in an amount of usually 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
In the present invention, the magnetic recording medium having the non-magnetic undercoat layer has the substantially same properties as those of the magnetic recording medium having no non-magnetic undercoat layer as described above. The magnetic recording medium having the non-magnetic undercoat layer according to the present invention can be readily surface-smoothened by calendaring treatment, and can be improved in running durability since a lubricant can be supplied from the non-magnetic undercoat layer.
Next, the process for producing the spindle-shaped magnetic alloy particles according to the present invention is described.
The spindle-shaped magnetic alloy particles of the present invention can be produced from spindle-shaped goethite particles containing cobalt in an amount of usually 20 to 50 atm % (calculated as Co) based on whole Fe and having an average major axis diameter of usually 0.05 to 0.15 xcexcm, or spindle-shaped hematite particles obtained by heat-dehydrating the above spindle-shaped goethite particles as starting particles by the following method. That is, the spindle-shaped magnetic alloy particles of the present invention can be produced by conducting a first step of heating the above starting particles to a temperature of usually 300 to 600xc2x0 C. in an inert gas atmosphere, and after replacing the inert gas atmosphere with a reducing gas atmosphere, heat-reducing the obtained particles at a temperature of usually 300 to 600xc2x0 C., thereby obtaining spindle-shaped magnetic alloy particles; a second step of subjecting the spindle-shaped magnetic alloy particles obtained in the first step to surface oxidation at a temperature of usually 80 to 200xc2x0 C. in an oxygen-containing inert gas atmosphere, thereby controlling the saturation magnetization value of the spindle-shaped magnetic alloy particles to 85 to 135 Am2/kg; a third step of heating the spindle-shaped magnetic alloy particles obtained in the second step to a temperature higher by not less than 50xc2x0 C. than the heat-reducing temperature used in the first step, which are between usually 400 to 700xc2x0 C., in an inert gas atmosphere, and after replacing the inert gas atmosphere with a reducing gas atmosphere, heat-reducing the obtained particles again at a temperature of usually 400 to 700xc2x0 C.; and a fourth step of subjecting the spindle-shaped magnetic alloy particles obtained in the third step again to surface oxidation at a temperature of usually 40 to 160xc2x0 C. in an inert gas atmosphere containing water vapor in an amount of usually 5 to 10 g/m3 and oxygen.
As described above, as the starting particles for spindle-shaped magnetic alloy particles of the present invention, there may be used spindle-shaped goethite particles containing cobalt in an amount of usually 20 to 50 atm % (calculated as Co) based on whole Fe and having an average major axis diameter of usually 0.05 to 0.15 xcexcm, or spindle-shaped hematite particles obtained by heat-dehydrating the above spindle-shaped goethite particles, which contain cobalt in an amount of usually 20 to 50 atm % (calculated as Co) based on whole Fe and have an average major axis diameter of usually 0.05 to 0.13 xcexcm.
The starting particles used in the present invention are spindle-shaped particles containing no dendritic particles and having an excellent size distribution.
The spindle-shaped goethite particles used as the starting particles may be produced from an aqueous ferrous salt solution and an aqueous alkali solution. As alkali contained in such an aqueous alkali solution, there may be used at least one selected from sodium carbonate solution, ammonium hydrogen carbonate solution and a mixed alkali solution composed of ammonium hydrogen carbonate and aqueous ammonia, or a mixed solution composed of the above-mentioned solution and sodium hydroxide solution. In the consideration of a less Na content and good magnetic properties of the obtained spindle-shaped goethite particles, it is preferred to use ammonium hydrogen carbonate and/or aqueous ammonia.
The surface of the spindle-shaped goethite particles used in the present invention may be coated with a Co compound, an Al compound or an anti-sintering agent.
As the anti-sintering agent, there may be used rare earth-containing compounds. Examples of rare earth elements contained in the anti-sintering agent may include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium or the like. These rare earth elements may be used alone or in combination of two or more thereof. Among these rare earth elements, yttrium is preferred.
In the spindle-shaped goethite particles, in order to further enhance the anti-sintering effect, there may be used, if required, one or more compounds containing other elements selected from Si, B, Mg, Ba, Sr and the like. These compounds have not only the anti-sintering effect, but also a function of controlling the reducing velocity and, therefore, may be used in any suitable combination according to requirements.
In the consideration of anti-sintering effect and well-controlled reducing velocity, the spindle-shaped goethite particles used in the present invention preferably have an average minor axis diameter of 0.010 to 0.024 xcexcm, an aluminum content of 5 to 15 atm % (calculated as Al) based on whole Fe, a rare earth content of 5 to 15 atm % (calculated as rare earth element) based on whole Fe, an aspect ratio of 4:1 to 8:1, and a BET specific surface area value of 100 to 250 m2/g.
Also, in the consideration of anti-sintering effect and well-controlled reducing velocity, the spindle-shaped hematite particles used in the present invention preferably have an average minor axis diameter of 0.010 to 0.023 xcexcm, an aluminum content of 5 to 15 atm % (calculated as Al) based on whole Fe, a rare earth content of 5 to 15 atm % (calculated as rare earth element) based on whole Fe, an aspect ratio (average major axis diameter/average minor axis diameter) of 4:1 to 8:1, and a BET specific surface area value of 50 to 120 m2/g.
The spindle-shaped hematite particles are preferably produced by heat-dehydrating the spindle-shaped goethite particles at a temperature of 150 to 350xc2x0 C. in an oxidative atmosphere, and then heat-treating the obtained particles at a temperature of from more than 450xc2x0 C. to less than 700xc2x0 C. in the same atmosphere.
Further, the thus heat-treated spindle-shaped hematite particles may be washed in order to remove impurity salts such as Na2SO4 derived from the production reaction of the spindle-shaped goethite particles therefrom. In this case, the removal of the impurities is preferably conducted by washing the spindle-shaped hematite particles under such a condition that the anti-sintering agent coated thereon is not eluted out.
In the present invention, when charging into a reducing apparatus, it is preferred to use as starting particles a granular powder having an average diameter of 1 to 5 mm.
As the suitable reducing apparatus used in the present invention, there may be exemplified such a reducing apparatus capable of forming a fixed bed of the particles. More specifically, there may be suitably used a stationary-type (batch-type) reducing apparatus or a movable-type (continuous-type) reducing apparatus capable of reducing the fixed bed formed on a belt while moving the belt.
In the present invention, the height of the fixed bed formed in the reducing apparatus is preferably not more than 30 cm. When the height of the fixed bed is more than 30 cm, there arises such a problem that the particles present in an upper portion of the fixed bed suffer from deterioration in coercive force by increase in water vapor partial pressure therein due to a remarkable reduction-accelerating effect by a large Co content and due to a rapid reduction reaction occurring at a lower portion of the fixed bed, resulting in deteriorated properties of the particles as a whole. In the consideration of industrial productivity, the height of the fixed bed formed in the reducing apparatus is more preferably 3 to 30 cm. Meanwhile, the production efficiency varies depending upon types of the reducing apparatus used, e.g., batch-type reducing apparatuses described in Japanese Patent Application Laid-Open (KOKAI) Nos. 54-62915 (1979) and 4-224609 (1992) or the like are different in production efficiency from continuous-type reducing apparatuses described in Japanese Patent Application Laid-Open (KOKAI) No. 6-93312 (1994) or the like. In the case of the batch-type fixed bed reducing apparatuses, the height of the fixed bed is preferably from more than 8 cm to 30 cm.
In the present invention, the atmosphere used during the heating period in the first and third steps for heating to the heat-reducing temperature is an inert gas atmosphere. As the inert gas for the inert gas atmosphere, there may be suitably used nitrogen gas, helium gas, argon gas or the like. Among these inert gases, nitrogen gas is preferred. When an atmosphere other than the inert gas atmosphere is used, the reduction is prematurely caused during the heating period at which the temperature is changed with the passage of time, so that the reduction temperature cannot be kept constant upon the production of the magnetic alloy particles, thereby failing to achieve a uniform crystallite growth in the particle and obtain particles having a high coercive force.
Meanwhile, the heating velocity in the first and third steps is not particularly restricted, and is preferably 2 to 100xc2x0 C./minute.
The superficial velocity in the inert gas during the heating period of the first and third steps is not particularly restricted, and may be determined so as to prevent the granular starting powder from being scattered or broken.
Also, in the heating of the first and third steps, the method of switching from the inert gas atmosphere to the reducing gas atmosphere for the heat-reduction process, varies depending upon kinds of reducing apparatuses used. From industrial viewpoint, in the case of batch-type reducing apparatus, the switching between the atmospheres is preferably carried out stepwise while controlling an inner pressure of the reducing apparatus, and in the case of continuous-type reducing apparatus, it is preferred that the reducing zone is separated from the heating zone. In any case, the switching between the atmospheres is preferably completed for a short period of time, specifically within 10 minutes.
The atmosphere used in the heat-reducing process of the first and third steps is a reducing gas atmosphere. As the reducing gas, hydrogen may be suitably used.
In the present invention, the heat-reducing temperature used in the first step is usually 300 to 600xc2x0 C., preferably 350 to 550xc2x0 C. The heat-reducing temperature of the first step may be appropriately selected from the above-specified range depending upon kinds and amounts of compounds used for coating of the starting particles. When the heat-reducing temperature is less than 300xc2x0 C., the reduction reaction may proceed very slowly in an industrially-unsuitable manner, so that the obtained spindle-shaped magnetic alloy particles may be deteriorated in saturation magnetization value. When the heat-reducing temperature is more than 600xc2x0 C., the reduction reaction may proceed more rapidly, so that the obtained particles may suffer from shape destruction or sintering within or between the particles, resulting in deteriorated coercive force thereof.
In the present invention, the superficial velocity of the reducing gas used in the first step is preferably 40 to 150 cm/second. When the superficial velocity of the reducing gas used in the first step is less than 40 cm/second, the water vapor produced by reduction of the starting particles may be discharged only too slowly out of the reaction system, so that the particles present in the upper portion of the fixed bed may be deteriorated in coercive force and SFD, thereby failing to obtain particles having a high coercive force as a whole. When the superficial velocity of the reducing gas used in the first step is more than 150 cm/second, although the aimed spindle-shaped magnetic alloy particles are obtained, there may be caused problems such as need of a higher reducing temperature, thereby scattering and breaking of the granulated product or the like.
In the second step of the present invention, the particles obtained in the first step are subjected to surface oxidation in an oxygen-containing inert gas atmosphere. As the inert gas used in the oxygen-containing inert gas atmosphere, nitrogen gas, helium gas, argon gas or the like are preferred. Among these inert gases, nitrogen gas is more preferred. The oxygen content of the oxygen-containing inert gas atmosphere is preferably 0.1 to 5% by volume. It is preferred that the amount of oxygen is gradually increased until reaching the aimed content.
In the present invention, the reaction temperature used in the second step is usually 80 to 200xc2x0 C., preferably 80 to 180xc2x0 C. When the reaction temperature used in the second step is less than 80xc2x0 C., it may be difficult to form a surface-oxidation layer having a sufficient thickness. When the reaction temperature used in the second step is more than 200xc2x0 C., the particles may suffer from change in skeleton thereof, especially extremely swelled minor axis diameter thereof due to the production of a large amount of oxides, resulting in destruction of skeleton of the particles in the worse case.
The spindle-shaped magnetic alloy particles obtained after completion of the second step has a saturation magnetization value of usually 85 to 135 Am2/kg (85 to 135 emu/g), preferably 90 to 130 Am2/kg (90 to 130 emu/g), more preferably 100 to 130 Am2/kg (100 to 130 emu/g). When the saturation magnetization value is less than 85 Am2/kg, the surface-oxidation layer is too thick, so that even though such particles are subjected to the heat-reduction process of the third step, it may be difficult to obtain spindle-shaped magnetic alloy particles having a high coercive force. When the saturation magnetization value is more than 135 Am2/kg, the formation of the surface-oxidation layer is insufficient, thereby failing to form a dense surface-oxidation layer.
Meanwhile, when whole part of the particles is oxidized in the second step, the particles suffer from change in skeleton thereof, especially minor axis growth, i.e., extremely swelled minor axis diameter due to the production of a large amount of oxides. As a result, since the skeleton of such particles is already destroyed, it is not possible to improve a coercive force thereof even by reducing the particles again.
The heat-reducing temperature used in the third step of the present invention is higher by not less than 50xc2x0 C. than the heat-reducing temperature used in the first step, and lies in the range of 400 to 700xc2x0 C. When the heat-reducing temperature used in the third step is not higher by not less than 50xc2x0 C. than the heat-reducing temperature used in the first step, or is less than 400xc2x0 C., the reduction reaction may proceed very slowly in an industrially-unsuitable manner, so that it may be difficult to reduce the surface-oxidation layer formed in the second step, and to densify the particles as a whole. When the heat-reducing temperature used in the third step is more than 700xc2x0 C., the obtained particles may tend to suffer from destruction of skeleton thereof or sintering within or between the particles, resulting in deterioration in coercive force thereof. The heat-reducing temperature used in the third step is preferably in the range of 450 to 650xc2x0 C.
In the present invention, the superficial velocity of the reducing gas used in the third step is preferably 40 to 150 cm/second similarly to the first step.
In the third step, the particles obtained after the reduction process step may be subjected to annealing treatment. The annealing treatment may be conducted at a temperature of preferably 500 to 700xc2x0 C. in a hydrogen gas atmosphere or an inert gas atmosphere such as especially nitrogen.
In the fourth step of the present invention, the particles are surface-oxidized in an inert gas atmosphere containing water vapor in an amount of 5 to 10 g/m3 and oxygen. When the water vapor content in the inert gas atmosphere is less than 5 g/m3, it may be difficult to form a dense and thin surface-oxidation layer, thereby failing to sufficiently improve a coercive force of the particles. When the water vapor content in the inert gas atmosphere is more than 10 g/m3, the effect of addition of water vapor is already saturated and, therefore, the addition of such a large amount of water vapor is unnecessary and meaningless. The water vapor content in the inert gas atmosphere is preferably 2 to 8 g/m3. The oxygen content in the inert gas atmosphere is preferably 0.1 to 5% by volume. It is preferred that the oxygen content in the inert gas atmosphere is gradually increased until reaching the aimed value. The inert gas contained in the above inert gas atmosphere is preferably nitrogen gas, helium gas, argon gas or the like. Among these inert gases, nitrogen gas is more preferred.
The reaction temperature used in the fourth step of the present invention is usually 40 to 160xc2x0 C., preferably 40 to 140xc2x0 C. Meanwhile, it is preferred that the reaction temperature used in the fourth step is lower than the surface-oxidation temperature used in the second step. When the reaction temperature used in the fourth step is less than 40xc2x0 C., it may be difficult to form a satisfactory surface-oxidation layer. When the reaction temperature used in the fourth step is more than 160xc2x0 C., the thickness of the surface-oxidation layer may be too large, resulting in deterioration in saturation magnetization value of the obtained particles.
The point of the present invention is that by using spindle-shaped magnetic alloy particles containing Fe and Co and having a specific crystallite size and a specific activation volume, it is possible to produce a magnetic recording medium having a less noise.
In the above production process of the present invention, the activation volume and the crystallite size of the particles can be controlled to small values by the reduction reaction and surface-oxidation of the first and second steps, and further a dense surface-oxidation layer can be formed by the reduction reaction and surface-oxidation of the third and fourth steps while minimizing the activation volume and the growth of crystallite size. As a result, it is considered by the present inventors that all of requirements including a high coercive force and a small rotational hysteresis integral value as well as a small crystallite size and a small activation volume can be achieved.
In addition, in the present invention, since the activation volume and the crystallite size are controlled to small values, it is possible to reduce the noise. In addition, the small rotational hysteresis integral value causes reduction in width of magnetization reversal, thereby approaching the magnetization reversal mode to coherent rotation. Further, since the spindle-shaped magnetic alloy particles of the present invention are fine particles having a high coercive force and a large anisotropy field, it is possible to obtain a high output performance.
Thus, since the spindle-shaped magnetic alloy particles of the present invention can satisfy various properties as described above, the magnetic recording medium using the spindle-shaped magnetic alloy particles according to the present invention can exhibit not only an excellent output characteristics in a short wavelength region and a considerably reduced noise, but also a small width of magnetization reversal. In addition, the spindle-shaped magnetic alloy particles produced by the above production process according to the present invention are free from sintering therebetween and can be provided thereon with a dense surface-oxidation layer, resulting in excellent dispersibility and oxidation stability. Also, the magnetic recording medium using the spindle-shaped magnetic alloy particles of the present invention is excellent in surface smoothness and long-term storage stability.
The spindle-shaped magnetic alloy particles of the present invention are small in major axis diameter, activation volume and crystallite size, and can exhibit a high coercive force, an excellent oxidation stability and a small rotational hysteresis integral value. Therefore, by using such spindle-shaped magnetic alloy particles, it is possible to produce a high-density magnetic recording medium having a high output performance and a less noise.
Thus, the magnetic recording medium produced using the spindle-shaped magnetic alloy particles of the present invention can be suitably used as a digital magnetic recording medium exhibiting a high-density recording property, high output characteristics and a less noise.